A Wide Area Network (WAN) is a data communications network that interconnects data processing equipment at separate locations which are geographically remote from each other, such as locations in different cities, states and countries. A wellknown and widely adopted WAN standard is CCITT (International Telegraph and Telephone Consultative Committee) Recommendation X.25. Recommendation X.25 is an international standard protocol for a WAN packet-switched network (PSN). Packet-switching involves splitting data into relatively small pieces, known as data packets, and adding network information, such as the source and destination address, to each packet. The X.25 protocol defines the organization of packets transmitted over the network.
To a user of an X.25 network, the network quite properly may be thought of as a central cloud surrounded by locations outside the cloud at which data terminal equipment (DTE) resides. Users of the DTEs at various locations communicate with each other via packets which are transmitted through the cloud. Inside the cloud are switching nodes and data communication links. A switching node is a data communication processor which routes data packets through network data communication links. Some data communication links, such as dedicated circuits and telephone lines, are used for connections between a user's equipment and a switching node. Other links, such as dedicated circuits, satellite channels and digital lines, are used to connect switching nodes. A separate network processor known as the network control center (NCC) performs network management, such as access control, failure tracking, billing, and so forth.
Operation of the cloud is completely transparent to users at DTEs. In fact, communication between two DTEs may be thought of as a single virtual circuit between the DTEs, despite the fact that individual packets transmitted over a single virtual circuit may follow different actual paths through the cloud. For each DTE outside the cloud, there is Data Circuit-terminating Equipment (DCE) inside the cloud. The DCE is the DTE's port into the network. A packet sent from a first DTE to a second DTE will enter the cloud at the first DTE-DCE interface. From the first DCE, the message is routed through the cloud to the second DCE. Following delivery at the second DCE, the packet finally exits the cloud at the second DTE-DCE interface. As far as the communicating DTEs are concerned, however, there is a single virtual circuit between them for reception and transmission of packets. Such DTE-DTE virtual circuits are possible, that is, the operation of the cloud is transparent to DTEs, because of a standard DTE-DCE interface. The X.25 WAN protocol defines the DTE-DCE interface standard.
The X.25 protocol divides DTE-DCE communication into three levels. X.25 at level-1 defines the mechanical, electrical, functional and procedural requirements for activating, maintaining and deactivating the physical DTE-DCE link. Level-1 communication consists of a synchronous bit stream and the fundamental unit of communication transfer is the BIT. Level-2 defines the procedure to access the DTE-DCE link and to allow error-free flow information between a DTE and its DCE. The fundamental unit of communication transfer at level-2 is the FRAME. A FRAME consists of a contiguous sequence of BITs. Level-3 defines the format for embedding both control information and user data in a single packet and the procedures for establishing, maintaining and clearing virtual circuits between DTEs. The fundamental unit of communication transfer at level-3 is the PACKET. A PACKET is a contiguous sequence of BITs embedded within a FRAME. There is only one PACKET in a FRAME.
In addition to the three levels of X.25 communication, there are divisions within level-2 and level-3. At level-2, there are nine types of FRAMEs which fall into one of three main categories: information FRAMEs (also known as I-FRAMEs), supervisory FRAMEs and unnumbered FRAMEs. Only I-FRAMEs contain level-3 PACKETs while the other two kinds of FRAMEs essentially contain network control information. Likewise, at level-3 there are seventeen types of PACKETs which may also be grouped according to network function.
The PACKET is the virtual level item of communication between DTEs. The X.25 defines a PACKET as a block of bytes having two parts, an n-byte header field followed by a data-field (although not every packet will necessarily have a data field). The data-field contains information comprising the DTE user's message. The data-field typically has a maximum length of 128 bytes. A single message usually consists of numerous PACKETs because of limitations on the data-field's length. The n-byte header field has at least three parts: a 4-bit general-format-indentifier field (GFI), followed by a 12-bit logical-channel-identifier field (LCI), followed by an 8-bit packet-type-indentifier field (PTI). The GFI, LCI and PTI contain important network information. The GFI contains, among other things, the type of acknowledgement required by the PACKET's sender. The LCI identifies the PACKET's virtual circuit at a given DTE-DCE interface. The 12-bit LCI consists of a 4-bit logical channel group number (LCGN) and an 8-bit logical channel number (LCN), which have further network signfificance. The PTI contains the PACKET's send and receive sequence numbers and identifies the PACKET's type. (The PACKET header may also include a DTE addressing field, depending upon the type of PACKET, and a facility field encoding the network facilities which the DTE user chose when subscribing to the X.25 network. The DTE addressing field encloses the network address of the "calling" and "called" DTEs. This field is present only in a so-called call-request PACKET which the X.25 network uses to initially set up a virtual circuit.) The X.25 WAN packet-switched network is explained in detail in the following references: X.25: The PSN Connection, An Explanation of Recommendation X.25, Hewlett-Packard, part no. 5958-3402, 1984; and, X. 25 Wide Area Networking for HP Computers, Hewlett-Packard Journal, Vol. 37, no. 10, p. 36, October 1986.
Since the LCI field of a Packet is 12-bits long it can identify 2.sup.12 =4096 separate channels, that is, DTE-to-DTE virtual circuits at a single DTE-DCE interface. (In X.25 systems, however, the first channel is always reserved for network control purposes so that at most 4095 channels are available at any one DTE-DCE interface.) Although the maximum number of simultaneously active channels at one DTE-DCE interface is normally less than one hundred, it is theoretically possible for all the channels to be active. In addition, channels may be grouped by type of service. Typically, a DTE user and the network will enter a local agreement at subscription time regarding assignment of LCI channels to type of service. Four types of service are available at a given DTE-DCE interface: one-way outgoing calls only, one-way incoming calls only, two-way and permanent virtual circuit. A call is essentially a virtual connection between two DTEs such that information data may be exchanged, conceptually analogous to a telephone call between two individuals. The potentially large number of active channels compounded by the different types of service creates a significant communications management problem for a DTE user at a busy DTE-DCE interface. Currently, we know of no prior art device which allows the DTE user to quickly determine the status of all 4095 possible channels at a given DTE-DCE interface.